This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Description: Progesterone antagonists (PAs) are compounds that block progesterone (P) action. Wyeth Pharmaceuticals is developing nonsteroidal PAs as therapy for endometriosis and uterine fibroids. Many of these compounds that act as pure P antagonists in rodents and human cell cultures, can switch functional profiles and act as pure progestin agonists in nonhuman primates and women in vivo. The mechanism that underlies profile switching in these different experimental models is not known. Our goal is to analyze the ability of these novel PA compounds to antagonize P action and or switch profiles in vivo in rhesus macaques. In this study we assessed the effects of W-891 (a nonsteroidal PR antagonist) in artificially cycled rhesus macaques.